Effect of contralateral sound stimulation on the distortion product 2F1-F2: evidence that the medial efferent system is involved.

The mechanical nonlinearity of the cochlea that is associated with normal cochlear function, induces distortion products that can be recorded in the external auditory canal. The acoustic cubic distortion product 2F1-F2 (DP) was measured in the external canal in the presence and in the absence of a contralateral white noise. The experiments were carried out on 20 guinea pigs after a section of the middle ear muscles. They showed that the presence of a contralateral white noise induces a significant reversible reduction of the magnitude of the DPs. This suppressive effect produced by the contralateral white noise was completely canceled out by the midline sagittal section of the brainstem. This report supports the hypothesis that the suppressive effect of a contralateral sound stimulation is mediated by the medial efferent system.